1. Field of the Invention
The present invention relates to an electrode plate for use in nonaqueous electrolyte secondary batteries typically including lithium-ion secondary batteries and to a nonaqueous electrolyte secondary battery using thereof.
2. Description of the Related Art
Recent years have seen rapid advances in miniaturization and weight reduction of electronic equipment and communication equipment. Thus, there has been a demand for miniaturization and weight reduction of secondary batteries for use as a driving power source in such equipment. For this purpose, in place of conventional alkaline storage batteries, there have been proposed nonaqueous electrolyte secondary batteries, typically lithium-ion secondary batteries, which can have a high energy density and a high voltage.
An electrode plate for use as a positive electrode of the nonaqueous electrolyte secondary battery (a positive electrode plate) is produced by using a complex oxide such as lithium manganate and lithium cobaltate as a positive active material, dispersing or dissolving the positive active material, an electrically-conductive material and a binder in an appropriate wetting agent (solvent) to prepare a slurry-like coating composition, and applying the coating composition onto a collector made of metal foil so that a positive active material layer is formed thereon.
On the other hand, an electrode plate for use as a negative electrode of the nonaqueous electrolyte secondary battery (a negative electrode plate) is produced by using a carbonaceous material such as carbon capable of occluding cation (such as lithium ion) as a negative active material, in which the cation is released from the positive active material at the time of charging, dispersing or dissolving the negative active material and a binder in an appropriate wetting agent (solvent) to prepare a slurry-like coating composition, and applying the coating composition onto a collector made of metal foil so that a negative active material layer is formed thereon.
A terminal for taking out electric current is then attached to each of the positive and negative electrode plates, both of which are then wound up with a separator (for preventing short circuit) sandwiched therebetween and sealed in a container filled with a nonaqueous electrolyte solution, so that a secondary battery is constructed.
In recent years, there has also been a demand for a nonaqueous electrolyte secondary battery with higher capacity, and various improvements have been made. An example of such improvements is to tightly wind the electrode plates for the purpose of increasing the amount of the active material to be stored in the limited volume, namely, a method of increasing the winding density of the electrode plates. However, an increased winding density can lead to a reduction in voids for infiltration of the liquid electrolyte, so that the liquid electrolyte can less infiltrate into the active material layer. Such reduced infiltration of the liquid electrolyte can lead to poor battery performance, even if the winding density of the electrode plate is increased for the purpose of increasing the capacity.
Japanese Patent Application Laid-Open (JP-A) No. 09-129223 discloses a method of ensuring paths for a liquid electrolyte by embossing the active material layer. In such a method, however, a load can locally be applied to projection portions of the embossed material, when the electrode plates are wound up. Thus, such a method can cause a rupture of the electrode due to expansion and contraction of the volume during charge and discharge cycles and has a problem with stability of quality.